1. Field of the Invention
This invention relates generally to a process for producing capacitors. More particularly, the present invention relates to a process for producing solid electrolytic capacitors of the type which comprises a lead wire partially embedded in and partially projecting from a compacted mass of powdered electrode material such as tantalum powder.
2. Description of the Prior Art
As is well known, capacitors have the function of storing and discharging electrical energy, and are widely used in designing electronic and electronical circuits. Obviously, the capacitor should preferably be small-sized but yet have a large capacity. One typical example meeting such requirements is the solid electrolytic capacitor.
The solid electrolytic capacitor is a polar capacitor which includes a metallic anode (positive electrode) and a solid electrolyte cathode (negative electrode). The surface of the metallic anode is oxidized to form an oxide layer acting as a dielectric substance separating electrically between the anode and the cathode. Most commonly used for the metallic anode is a compacted mass of tantalum powder.
To specifically explain the problems to be solved by the present invention, reference is now made to FIG. 5 schematically showing a typical prior art process for producing a solid electrolytic capacitor which incorporates a tantalum electrode. As shown in FIG. 5, tantalum powder 11 is first loaded into a molding recess 10a of a mold 10, and a short lead wire 12 is inserted into the load of tantalum powder. Then, the load of tantalum powder is compacted by a presser rod 13. The resulting compact 11' is removed from the mold. Obviously, the compact 11' is highly porous and therefore has a large surface area (including the internal pore surface area) which increases the capacity of the resulting capacitor.
The compact 11' of tantalum powder thus obtained is sintered in a vacuum sintering furnace (not shown). Thereafter, the compact is dipped in a chemical solution bath (not shown) and electrolytically oxidized to form an oxide coating or layer (Ta.sub.2 O.sub.5) which is a dielectric substance. The electrolytic oxidation occurs within the pores of the compact according to the following reaction. EQU Ta.sub.2 +5H.sub.2 O.fwdarw.Ta.sub.2 O.sub.5 +5H.sub.2
After the chemical treatment described above, the compact is impregnated with a solution of manganese nitrate (Mn(NO.sub.3).sub.2) which is thermally decomposed to form a layer of manganese dioxide (MnO.sub.2) which is a solid electrolyte substance. The manganese dioxide layer acts as a first cathode layer.
Then, the compact is subjected to a graphitizing treatment and a silver coating treatment to form second and third cathode layers on the manganese oxide layer. A capacitor element is thus obtained.
Finally, the capacitor element is formed into a final product by undergoing aftertreatments which include a soldering step, and encasing step, a marking step, and etc.
The prior art process described above is disadvantageous in the following respects.
First, according to the prior art process, only a single compact is obtained by a single compacting step, so that the production efficiency is very low. It is conceivable to improve the production efficiency by using a plurality of molds for simultaneously producing a corresponding number of compacts at a time. In this case, however, it is still necessary to separately prepare a corresponding number of short lead wires and introduce them in the respective molds in addition to separately loading the molds with tantalum powder, consequently failing to provide a reasonable increase of the productivity.
Second, tantalum powder is compacted by applying a pressing force axially of the lead wire 12. Thus, during the compacting step, the portion of the lead wire inserted in the load of tantalum powder may be subjected to buckling which may deteriorate the quality of the product. This problem is particularly serious when the lead wire is very thin, but the lead wire should be as thin as possible to realize miniaturization of the capacitor. Indeed, the prior art process is applicable only to lead wires of no less than 180 .mu.m in diameter.